This research proposal concerns the molecular pathogenesis of disease caused by flaviviruses. These viruses include many agents of arthropod- transmitted encephalitis and hemorrhagic fever which cause serious human illness on a global scale. Prevention of such diseases with live- attenuated and inactivated vaccines is an important public health priority but progress in this area requires better knowledge of the molecular basis of viral virulence and the requirements for induction of protective immunity. Our long range objectives are to understand at the molecular level the interactions between virus and host during pathogenesis. A major focus is on identifying which viral proteins are virulence factors. This question is being explored using a molecular clone of yellow fever virus (YF) to study pathogenesis in a mouse model of acute encephalitis. The specific aims include defining the genetic determinants within a neuroadapted yellow fever strain which account for its high neurovirulence for normal mice and its high neuroinvasiveness in SCID mice. The second major focus is on the process by which protective immune responses are elicited within the central nervous system during acute flavivirus encephalitis. This question is being approached using a model in which clearance of yellow fever virus occurs from the central nervous system of previously immunized mice. The patterns of expression of cytokine genes within the brain are being analyzed as a means of characterizing the immune response associated with protection. The specific aim is to determine how cytokine expression differs between immunized animals which do not experience disease and unimmunized animals, which develop fatal encephalitis. This work is expected to provide insight into understanding both the mechanisms by which an antiviral state is induced within the brain and the immunoregulatory events responsible for resolution of the inflammatory response in this compartment. Results of this investigation may influence strategies for the development of flavivirus vaccines and be relevant to understanding other infectious diseases of the central nervous system.